Originally Posted by
mad_jock
Its the only way it can work.
As a matter of interest Genghis how do they actually work out the MTOW of a light aircraft?
In many ways, you go around in circles. It works something like this:
- First work out your design spec (performance, payload, internal size, equipment)
- Using charts and tables that are readily available in aircraft design textbooks, come up with rough figures for the masses of the major subsystems (wings, fuselage, cockpit, gear, powerplant...)
- Now start refining it by using some real components, particularly the engine, as there are only so many engines available and this tends to start dictating everything else.
- Go round the loop a few times to come up with more accurate estimates of size, shape, component list.
- Take the estimated empty weight and payload, add them up, add a bit more, and set this as your provisional MTOW.
- Now think through any later modifications you might want to make, what those will weigh, and add a bit more onto the MTOW.
- Outline design the main structural components (gear, mainplane, fuselage) to the MTOW, powerplant structure to the engine, tail feathers to match the performance, desired CG range, MTOW.
- Revise the empty weight estimates, decide whether you need to revise the MTOW, payload and performance targets at-all. The odds are you'll go around this loop quite a few times before you're reasonably happy that all the numbers line up.
- Now do the detailed design, build a structural prototype, and plan a set of physical tests - particularly on the mainplain (+Ve and -Ve g), engine mounts (torque, flight envelope, thrust, crash loads), any fuel tanks not built into in the wings, all of the control surfaces (equivalent of full deflection at Vc, 1/3 deflection at Vne), fin (max designed sideslip at Vne), horizontal stabiliser (MTOW, Fwd CG, Vne is usually the critical case). Based upon the test results some redesign might be needed, or some small changes to the MTOW might be needed.
- Finally, declare MTOW, build the flight test prototype, and then there will be some confirmation of MTOW compared to performance requirements. In practice however, very few aeroplanes - maybe the odd underpowered light twin or very old / small engined microlight are MTOW limited by performance - although I've met one or two. SEIO climb performance on twins, or mimimum time to 1000ft at ISA S/L standard conditions are generally the factors you're checking at. On a microlight or VLA designed too close to the stall speed limits, I suppose it's possible that too high a stall speed can also force reduction of MTOW, but that was a serious design cock-up if it does.
You weren't looking for a quick and simple answer I hope MJ?
G